1. Field of the Invention
The invention relates to apparatus for the control of a salient-pole machine.
2. Description of the Prior Art
U.S. Pat. No. 3,775,649 describes a device for controlling a converter-fed, automatically controlled synchronous machine in which the desired value for the stator current (regarded as a vector) is preset in the form of a component parallel to the flux vector and a component normal thereto. In this field-oriented system, the field-parallel component determines the contribution of the stator current to the magnetizing current and can, in particular, be set to zero. For a given flux, the field-perpendicular component determines the electrical torque furnished by the machine and can, in particular, be used, for controlling or regulating the speed of rotation or the torque.
To this end, the desired stator current vector preset by these "field-oriented" components is transformed in a vector rotator according to the angle (flux angle .phi..sub.s) between the flux vector and a fixed stator axis into a fixed ("stator-oriented") coordinate system, and is divided over the machine terminals, so that the stator current can now be controlled or regulated to this desired vector. The information about the flux angle .phi..sub.s is formed by a flux computer, which is fed by the actual value of the rotor position angle .lambda..sub.s, as well as by the actual values for the field current and for the stator current. The flux computer solves the differential equations of the electrical part of the machine using fed-in machine parameters for the rotor resistance and for the main inductance. In the case of a nonsalient-pole synchronous machine, whose main inductance is rotation-symmetrical, only one inductance parameter is needed. In the case of a salient-pole machine, the relationship between the flux and the current of the machine, given by the main inductance, is dependent on the relative position of the flux to the rotor magnetic axis and must therefore be separated into a series inductance acting on the flux component parallel to the rotor axis and a shunt inductance acting on the perpendicular flux component. Thus, calculations to solve the aforementioned diffential equations must be carried out using a main inductance which varies with the flux angle. The flux computer described in U.S. Pat. No. 3,775,649 constitutes a mathematical model of the machine and operates in the rotor-oriented reference system, in which the field current is parallel to the rotor- or d-axis. In principle, the flux vector may also be picked up directly by Hall probes or may be determined using another mathematical model, such as is described e.g. in FIG. 5 of commonly-owned, copending U.S. Pat. app'n Ser. No. 279,352, now U.S. Pat. No. 4,388,577, entitled "Rotating Field Machine Drive", filed July 1, 1981. This model operates in the field-oriented coordinate system with a uniform main inductance, and therefore applies only to nonsalient-pole machines. Another possibility for determining the flux, also described in U.S. pat app'n Ser. No. 279,352, now U.S. Pat. No. 4,388,577 is to calculate the EMF of the machine from motor voltage and motor current and to obtain the flux therefrom by integration ("voltage model"). Not only is it possible to use the information furnished by these mathematical models about the flux vector for field-oriented regulation of the stator current, but the model flux magnitude can be made use of for maintaining the flux magnitude at a constant desired value through regulation of the field current.
It has been proposed in U.S. Pat. No. 3,896,351 to precontrol the field current instead of, or in addition to, regulating the flux. By dividing the desired flux value by the main inductance and subtracting the field-parallel portion of the desired stator current vector, the field-parallel component i.sup.E* .multidot.cos .phi..sub.L of the desired field current vector is formed, which (vector) after division by cos .phi..sub.L determined by the mathematical model constitutes a control input i.sup.E* for the magnitude of the field current. By this anticipatory control, therefore, the field current is available at the start in such a way that, considering the magnetizing portion i.sub..phi.1.sup.S of the stator current, the total flux flow is already made to follow the value belonging to the present desired flux value .psi.*. If from the model flux and the desired flux magnitude there is formed additionally the control deviation of a flux control, this control serves only to make the field current follow with high dynamic accuracy also in the case of rapid changes.
The relationship i.sup..mu. =X.multidot..psi. utilized for the magnetizing current i.sup..mu. (field-parallel component of the total flux flow), main inductance X and flux magnitude .psi. is valid only for constant inductance, so that such anticipatory control cannot be used for a salient-pole machine for which the main inductance is asymmetrical.